A wind turbine generally has several blades connected to a hub, so that the blades cause the hub to rotate during operation. To alter the angle of pitch of a blade, it can be connected to the hub using a pitch bearing. A pitching system, usually installed in the hub, can drive the pitch bearing to adjust the pitch of the blade. In prior art systems, the pitch bearing is a roller or ball bearing. The roller bearings must be precisely machined using a high quality steel in order to ensure a satisfactorily long lifetime. The race is sealed to keep the lubricant in and moisture out. In order to replace a damaged bearing, the blade must be removed. This can be very time-consuming and expensive. While the bearing is being repaired, the blade must be supported, for example by a crane.
However, the increasing size of wind turbines and increasing trend toward offshore turbines puts high demands on serviceability and robustness that the prior art roller bearing systems cannot satisfy. A roller bearing is only practicable for blades up to a certain length and/or weight, since the load exerted on the bearing by a very long and/or heavy blade is correspondingly high. The large dynamic loads and corresponding structural deflections combined with strict requirements for quality and handling make the prior art roller bearings problematic and costly. Furthermore, the necessity of a crane to support a blade while the roller bearing is being repaired is even more costly for an offshore turbine.